This invention relates to methods of providing precursors for use in image-forming methods, for example to make lithographic printing forms or electronic parts, such as printed circuits. The invention relates further to such precursors per se, and to their use.
The art of lithographic printing is based on the immiscibility of ink, generally an oily formulation, and water, wherein in the traditional method the ink is preferentially retained by the image or pattern area and the water or fountain solution is preferentially retained by the non-image or non-pattern area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water while the image area accepts ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
New types of "waterless" lithographic printing employ only an oily ink material and preferentially ink-accepting image areas and ink-repelling non-image areas on the printing form.
A generally used type of lithographic printing form precursor (by which is meant a coated printing form--or plate--prior to exposure and development) has a radiation sensitive coating applied to an aluminium substrate. Negative working lithographic printing form precursors have a radiation sensitive coating which when imagewise exposed to radiation of a suitable wavelength hardens in the exposed areas. On development the non-exposed areas of the coated composition are removed leaving the image. In contrast, positive working lithographic printing form precursors have a radiation sensitive coating, which after imagewise exposure to radiation of a suitable wavelength becomes more soluble, in a developer, in the exposed areas than in the non-exposed areas. In both cases only the image area on the printing form itself is ink-receptive.
The differentiation between image and non-image areas is made in the exposure process where a film is applied to the printing form precursor with a vacuum to ensure good contact. The printing form precursor is then exposed to a radiation source; conventionally this has been a UV radiation source. In the case where a positive printing form precursor is used, the area of the film that corresponds to the image in the printing form precursor is opaque so that no light will strike the printing form precursor, whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating which becomes more soluble and is removed on development.
The resists used in pattern forming methods for electronic parts such as printed circuits are also classified into two types: negative working and positive working. After exposure to radiation and development, the resist pattern is used as a mask for forming the patterns onto the underlying electronic elements--for example by etching an underlying copper foil. Due to the high resolution demands and the requirements of high resistance to etching techniques, positive-working systems are widely used. In particular, in the main there have been used alkali developable positive working resists mainly composed of alkali-soluble novolak resins.
The types of electronic parts whose manufacture may use a resist include printed wiring boards (PWBs), thick- and thin-film circuits, comprising passive elements such as resistors, capacitors and inductors; multichip devices (MDCs); and integrated circuits (ICs). These are all classified as printed circuits.
Imagable compositions may also be applied to masks. The required pattern is formed on the mask, which is then used as a screen in a later processing step, in forming a pattern on, for example, a printing or electronic part precursor.
Common to virtually all commercial applications of positive working systems employing UV radiation over several decades have been compositions comprising alkali soluble phenolic resins and naphthoquinone diazide (NQD) derivatives. The NQD derivatives have been simple NQD compounds used in admixture with resins, or NQD resin esters in which the photoactive NQD moiety has been chemically attached to the resin itself, for example by esterification of the resin with an NQD sulphonyl chloride.
As demands on the performance of UV sensitive positive working coatings have increased so NQD technology has become limiting. In addition, digital and laser imaging technology is making new demands on the coatings.
It is known from GB 1245924 that the solubility of phenolic resins in developers may be increased by the application of heat. The heat may be delivered by infra-red radiation, assisted by radiation absorbing components such as carbon black or Milori Blue (C.I. Pigment Blue 27). However the developer resistance of the non-exposed areas to commercial developers is low, and the solubility differential generated on application of heat is low compared to the commercial UV sensitive compositions containing NQD moieties.
U.S. Pat. No. 5,372,915 is an example of a printing form containing a radiation sensitive composition which is comprised of a resole resin, a novolak resin, a latent Bronsted acid and an infrared absorber. In the preparation of a lithographic printing form, the radiation sensitive composition is imagewise exposed to activating infrared radiation and the exposed areas of the printing form are removed with an aqueous alkaline developing solution. Related U.S. Pat. No. 5,340,699 discloses the preparation of a lithographic printing form using the same radiation sensitive composition as in U.S. Pat. No. 5,372,915. But in this related patent the radiation sensitive composition is imagewise exposed to activating radiation, and then the printing form is heated to provide reduced solubility in exposed areas and increased solubility in unexposed areas. The unexposed areas of the printing form are then removed with an aqueous alkaline developer. Although the composition is the same, a positive or a negative lithographic image is produced in accordance with each respective patent by varying the activating radiation and adding a blanket heating step.
Further examples of radiation sensitive compositions and their use in making lithographic printing forms are disclosed in U.S. Pat. Nos. 4,708,925; 5,085,972; 5,286,612; 5,441,850; 5,491,046; 5,340,699; 5,466,557; and 5,372,907; European patent application 672 954 A2; and WO 96/20429.
New positive working heat sensitive systems have been developed to meet the new demands, as exemplified by: WO 97/39894; WO 99/01796; WO 99/01795; WO 99/08879; and WO 99/21725. It has been observed that in these new systems there may be an alteration in their sensitivity over time, after the heat sensitive composition has been applied to a substrate and dried, such effect being the result of reduced developer solubility of the unexposed compositions with time prior to exposure. Thus when the term "sensitivity" is used herein, it is used in the context of the entire process of exposure and development, and is not referring to the matter of how the areas of the composition which are exposed react to that exposure. Sometimes this "sensitivity" is called "operating speed" in the lithographic printing art.